This section provides background information related to the present disclosure which is not necessarily prior art.
Clutch assemblies are commonly used in vehicle transmissions to shift into a forward gear, to shift between forward gears, or to shift into a reverse gear. As a shift is being effectuated by the transmission, clutch assemblies selectively supply torque from a prime mover, such as an engine, to one or more gears of the transmission. Because clutch assemblies can decouple the prime mover from the one or more gears of the transmission, shifts can be accomplished without stalling the engine and/or binding the transmission.
Such clutch assemblies may generally include a clutch housing and a shaft that is disposed within the clutch housing. A clutch pack may be disposed radially between the clutch housing and the shaft. The clutch pack selectively couples rotation of the clutch housing and the shaft. The clutch pack may include various combinations of friction plates and reaction plates that transfer torque between the clutch housing and the shaft when the friction plates and reaction plates are pressed together in an engagement direction by a clutch actuator. While the friction plates and the reaction plates are rotatably coupled to the clutch housing and the shaft, they are moveable in the engagement direction with respect to the clutch housing and the shaft. A backing plate may be provided at one end of the clutch pack that is retained on either the clutch housing or the shaft. Longitudinal movement of the backing plate relative to the clutch housing and the shaft may be limited such that the backing plate acts as a stop for the friction plates and reaction plates when the friction plates and reaction plates are pushed in the engagement direction towards the backing plate by the clutch actuator. A single apply plate is provided at an opposite end of the clutch pack. Like the friction plates and reaction plates, the single apply plate is moveable in the engagement direction with respect to the clutch housing and the shaft.
The clutch actuator may be configured to apply pressure to the single apply plate in the engagement direction. This unidirectional pressure causes the single apply plate to slide towards the backing plate in the engagement direction. As a result, the friction plates and the reaction plates of the clutch pack are squeezed between the single apply plate and the backing plate. In other words, actuation of the clutch actuator longitudinally compresses the clutch pack in the engagement direction. Torque transfer between the friction plates and the reaction plates then occurs through friction interfaces that are disposed between adjacent friction plates and reaction plates. The clutch actuator may release the unidirectional pressure applied to the single apply plate to disengage the clutch pack. When the clutch actuator releases the unidirectional pressure applied to the single apply plate, the single apply plate moves longitudinally in a disengagement direction. The disengagement direction is generally opposite the engagement direction such that the single apply plate moves away from the backing plate in the disengagement direction, allowing the clutch pack to longitudinally expand.
The clutch engagement process begins when the clutch actuator applies pressure to the single apply plate. Where the clutch assembly is a wet clutch assembly, the clutch engagement process may generally be described in three stages: the hydrodynamic stage, the squash stage, and lock-up stage. The hydrodynamic stage is the first stage in the clutch engagement process. While the clutch actuator is moving the single apply plate longitudinally toward the backing plate in the hydrodynamic stage, there is little to no contact between the friction plates and the reaction plates of the clutch pack and the friction plates and reaction plates remain separated by a fluid film. Typically, no torque is transferred between friction plates and the reaction plates and thus the clutch shaft and the housing in the hydrodynamic stage. However, it should be appreciated that small, incidental amounts of torque transfer may occur in the hydrodynamic stage through fluid shear occurring within the fluid film disposed between adjacent friction plates and reaction plates. The squash stage is the second stage in the clutch engagement process. The clutch actuator is still moving the single apply plate longitudinally toward the backing plate in the squash stage, but now the friction plates have begun to contact the apply plates. In the squash stage, there is slippage (i.e. relative motion) between the friction plates and the apply plates despite the friction plates contacting the reaction plates at the friction interfaces. The kinetic energy of the relative motion between the friction plates and the reactions plates is absorbed during slippage and converted to friction generated heat. Although this heat is undesirable from a thermal management standpoint, this slippage is necessary to allow for the gradual transfer of torque between the shaft and the housing without stalling the prime mover, shocking the clutch assembly (which could lead to structural failures), and rapid, jerky acceleration. The majority of the torque transferred through the clutch assembly during the squash stage is done at the points of material contact at the friction interfaces between the friction plates and the reaction plates with tribo-chemical, mixed lubrication and/or elasto-hydrodynamic lubrication layers and not through fluid shear. The lock-up stage is the third stage in the clutch engagement process. In the lock-up stage, the clutch actuator is still applying pressure to the single apply plate in the engagement direction, but the single apply plate has stopped moving longitudinally toward the backing plate because the clutch pack is fully compressed between the single apply plate and the backing plate. In the lock-up stage, there is no slippage (i.e. relative motion) between the friction plates and the reaction plates such that the shaft rotates with the clutch housing and 100 percent of the torque at the shaft is transferred to the clutch housing. Accordingly, in the lock-up stage, there is no heat generation between the friction plates and the reaction plates and the friction interfaces begin to cool.
Temperature as it relates to the torque being transmitted through the clutch assembly is a primary design consideration when selecting the size, number, and material of the friction plates and the reaction plates. In dual-clutch transmissions in particular, temperature build-up in the friction interface(s) near the clutch actuator is a primary limiting factor. Temperature build-up in the clutch assembly limits toque capacity and is also a major consideration when designing clutch cooling components. In the case of wet clutches, where the friction plates and reaction plates are immersed in fluid, selection of the viscosity and formulation of the fluid, the fluid capacity of the clutch housing, and the pumping capacity of the clutch assembly are closely tied to the expected temperature build-up. Temperature build-up in the clutch assembly occurs when the clutch actuator begins to compresses the clutch pack, which produces friction generated heat as the friction plates and the reaction plates contact one another. The unidirectional pressure applied to the clutch pack in the engagement direction leads to variation in the time the friction plates are compressed in contact with the reaction plates. This results in temperature variations among the friction interfaces, with the highest temperatures occurring at the friction interfaces that have been in contact the longest (i.e. the friction interfaces closest to the clutch actuator). The temperature build-up in clutch assemblies has been studied in detail. Authors Ten et al. published one such study, entitled “Thermal analysis of a wet-disk clutch subjected to a constant energy engagement,” in the International Journal of Heat and Mass Transfer, Volume 51, Issues 7-8, April 2008, Pages 1757-1769. This study confirms that the highest temperatures in clutch assemblies typically occur at the friction interfaces nearest the clutch actuator. Accordingly, the friction interfaces nearest the clutch actuator act as a design constraint and the overall clutch assembly must be designed to avoid overheating of these friction interfaces.
In many clutch assemblies, peak temperatures occur at the friction interface that is the second closest to the clutch actuator. This is attributed to the fact that the single apply plate, clutch actuator, and clutch housing can act as a heat sink, cooling the friction interface that is closest to the clutch actuator. As a result, the friction interface that is second closest to the clutch actuator is often the one that overheats and is therefore a primary limiting factor in the design of clutch assemblies. To reduce the likelihood of overheating the friction interfaces nearest the clutch actuator, oversized friction plates may be used that have greater thickness and/or larger diameters. The increased mass and/or surface area of the oversized friction plates improves heat dissipation away from the friction interfaces and also makes the friction plates less prone to heat related failures. However, the size increase of the friction plates negatively impacts the efficiency, packaging, and price of the clutch assembly. Another way overheating is addressed is by increasing the fluid capacity of the clutch housing and/or pumping capacity of the clutch assembly. While increasing these parameters provides better cooling to the friction interfaces, greater fluid capacity and pumping capacity negatively impacts efficiency, packaging, and price of the clutch assembly.
Another approach for reducing the likelihood of overheating the friction interfaces nearest the clutch actuator is to control clutch actuation and/or the prime mover such that reduced torque is transmitted through the clutch assembly. In some instances, actuation of the clutch assembly may be controlled so as to provide for early clutch engagement before the amount of torque transmitted through the clutch assembly is high (i.e. early lock-up). In other instances, the torque transmitted through the clutch during vehicle launch may be reduced by launching the vehicle in second gear instead of in first gear. As disclosed in U.S. Pat. No. 6,095,946 to Maguire et al., another control method is to limit the output of the engine during clutch engagement so that repeated shifts will not overheat the friction interface. For example, fuel to the engine may be limited or the spark timing may be retarded to reduce the amount of torque that the engine supplies to the clutch assembly. A major drawback to these approaches however is that driving quality is negatively impacted. Acceleration and power may be compromised under these approaches and shift quality is reduced. Noise, vibration, and harshness (NVH) is often increased under such control schemes, which negatively affects customer driving experience. What is needed is a solution that reduces the likelihood of overheating the friction interfaces without the associated efficiency, packaging, cost, and drive quality drawbacks noted above.